Viki Bankey Alejandro Cuevas David Daniels Carol A. Finn Israel Hernandez Patricia Hill Robert Kucks Warner Miles Mark Pilkington Carter Roberts Walter Roest Victoria Rystrom Sarah Shearer Stephen Snyder Ronald Sweeney Julio Velez Phillips, J.D. Ravat, D. 2002 raster digital data U.S. Geological Survey Open-File Report 02-414 Denver, Colorado, USA U.S. Geological Survey This citation generally refers to the DVD and USGS web site that presents the digital gridded data used to make the North American magnetic map. Modifications have been made to reflect the use of this on the Mineral Resource Spatial Data website (mrdata.usgs.gov) in such areas as projection and image resolution. The image and grid available on http://mrdata.usgs.gov/ is the one referred to as USmag_hp500.grd. http://pubs.usgs.gov/of/2002/ofr-02-414/ A digital magnetic anomaly database and map for the North American continent is the result of a joint effort by the Geological Survey of Canada (GSC), U. S. Geological Survey (USGS), and Consejo de Recursos Minerales of Mexico (CRM). The database and map represent a substantial upgrade from the previous compilation of magnetic anomaly data for North America, now over a decade old. This integrated, readily accessible, modern digital database of magnetic anomaly data will be a powerful tool for further evaluation of the structure, geologic processes, and tectonic evolution of the continent and may also be used to help resolve societal and scientific issues that span national boundaries. The North American magnetic anomaly map derived from the digital database provides a comprehensive magnetic view of continental-scale trends not available in individual data sets, helps link widely separated areas of outcrop, and unifies disparate geologic studies. Publicly available airborne and marine magnetic data have been collected in North America primarily by the governments of Canada, the U.S., and Mexico. USmag_origmrg.grd Geosoft grid (GRD) file The residual total intensity of the Earth's magnetic field USmag_hp500.grd Geosoft grid (GRD) file 500-km high-pass filtered grid calculated from residual magnetic grid USmag_CM.grd Geosoft grid (GRD) file The result of removing a magnetic field that was calculated from satellite magnetic data by Tiku Ravat of Southern Illinois University. 1945 2001 Data collection period complete none planned -128.5 -65.0 49.5 23.3 none United States Mexico Canada Atlantic Ocean Pacific Ocean none aeromagnetic data geophysical data aeromagnetic grid North American magnetic map none none Robert P. Kucks U.S. Geological Survey mailing address

Box 25046, Mail Stop 964 Denver Federal Center

Denver CO 80225-0046 USA 303-236-1405 rkucks@usgs.gov ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/open-file-reports/ofr-02-0414/NAmag.jpg low-resolution graphic file of 1000-meter merged gridded data jpeg North American Magnetic Anomaly Group (NAMAG) >Viki Bankey (USGS) >Alejandro Cuevas (CRM) >David Daniels (USGS) >Carol A. Finn (USGS) >Israel Hernandez (CRM) >Patricia Hill (USGS) >Robert Kucks (USGS) >Warner Miles (GSC) >Mark Pilkington (GSC) >Carter Roberts (USGS) >Walter Roest (GSC) >Victoria Rystrom (USGS) >Sarah Shearer (USGS) >Stephen Snyder (USGS) >Ronald Sweeney (USGS) >Julio Velez (CRM) >Jeffrey Phillips (USGS) >D.K.A. Ravat (Southern Illinois University) These grids were created using the proprietary software Oasis Montaj, from Geosoft, Inc., and are in Geosoft grid format. A free Geosoft interface is available at: http://www.geosoft.com/pinfo/free/ This interface can export grids to many other common formats. U.S. Geological Survey National Geophysical Data Center 2002 digital aeromagnetic profile data U.S. Geological Survey Open-File Report 02-361 Denver, CO, USA U.S. Geological Survey Report contains digital aeromagnetic profile data from airborne surveys flown in the United States for the USGS. These data were processed from the original digital data provided by the contractor or the USGS. Data are available either on a DVD or online at the address below. http://pubs.usgs.gov/of/2002/ofr-02-0361 U.S. Geological Survey 1999 xyz ASCII data U.S. Geological Survey Open-File Report 99-557 Denver, CO U.S. Geological Survey The data available in this report are a compilation of digitally converted analog magnetic field intensity maps from the USGS or other public sources. http://pubs.usgs.gov/of/1999/ofr-99-0557/html/mag_home.html Grid values represent the total intensity of the Earth's magnetic field after removal of the International Geomagnetic Reference Field (IGRF). Measurements were made using a variety of magnetometer systems with typical accuracies of 1 to 10 nanotesla (nT). The grids presented in this report were made from numerous individual grids that were mathematically merged together using standard techniques. Individual metadata files exist for USGS gridded data created from digital flight-line data (see USGS Open-File Report 02-0361). The data in the original grids have been processed using formulas and methods that are not usually documented but that represent industry standard practices for airborne data reduction. The data in this file were collected by various contractors or groups who were responsible for collecting and processing the data. The data used to make these grids were collected using different instruments (magnetometers, altimeters, navigational systems) throughout the survey, but were usually consistent within each survey. Survey contracts specified the conditions and specifications under which these data were collected. Standard industry practices of the time were followed in data collection and processing. The North American Magnetic Anomaly grids were compiled from the best available aermomagnetic surveys publicly available in 2002. Best available would be data having the highest resolution, closest line-spacing, most accurate instrumentation, most accurate flight-elevation surface, and similar specifications commonly used in the airborne surveying industry. In many areas, only one survey was available. In many offshore areas and some onshore areas, no data were available at all, and these areas were left blank. Flight Path Recovery - before about 1975 Horizontal position of the survey aircraft used to collect data were determined by reconciling down-looking photographs (recorded on continuous-strip film) with topographic maps and orthophotoquadrangle maps. Fiducial numbers and marks, impressed on any paper strips that were recording data or added to magnetic tape records, were included as a function of time to further reconcile location with instrumentation. Flight Path Recovery - before about 1990 Horizontal position of the survey aircraft used to collect data were determined using aircraft navigational aids such as line-of-sight electronic systems that measure the distances from each of two ground stations to the aircraft using microwave or radio transmitters. Flight Path Recovery - after about 1990 Horizontal position of the survey aircraft used to collect data were determined using GPS satellite navigation. The aircraft vertical position was determined using the navigational positioning equipment on the aircraft, which were radar altimeter and barometric altimeter. Radar altimeters are estimated to have an error of 2-5% of the altitude (Richard Hansen, PRJ, Inc., written communication). Barometric altimeters are quite accurate, but are typically operated in an uncorrected mode. The diurnal variation in air pressure over the course of a flight can produce a 50-100 ft error in the barometric altimeter reading. In addition, pressure microcells create short-period air pressure changes equivalent to about 10 ft under typical conditions (Richard Hansen, PRJ, Inc., written communication) The magnetometer was carried on a "stinger" that was attached to the aircraft or was carried in a bird towed on a line that was below the aircraft. The bird as it is towed is slightly behind the aircraft and therefore the vertical distance between the magnetometer and the aircraft is slightly less than the length of the line but remains constant for the survey. Data were either collected at a fixed barometric altitude or were collected as a draped survey having an average terrain clearance above the ground. Because aircraft, especially airplanes, cannot safely maintain a constant terrain clearance, error in vertical position is introduced. We obtained all available aeromagnetic surveys. Some data were digitized from analog maps if digital data were unavailable. The remainder were processed using digital flight-line data. Details of data acquisition are described in the booklet that accompanies the printed map. The marine data were obtained from the National Geophysical Data Center of the National Oceanic and Atmospheric Administration spanned the years 1958 through 1997. 2002 Aeromagnetic data were processed so that residual magnetic values were calculated in a consistent manner: - The DGRF, updated to survey date, was removed - The flight elevation was used to mathematically calculate the equivalent magnetic field at 1,000 ft. above terrain - obvious errors were corrected - x-y locations were calculated for the described DNAG projection - data were gridded at 1/3 - 1/4 of the flight-line spacing, then regridded to 1 km. 2002 Using various USGS in-house programs (before 1999) or Geosoft/Oasis Montaj (after 1999), gridded data were mathematically merged together. For the U.S. part, scientists merged groups of surveys by state or groups of states. States were merged into regions, and regions into countries. For progress on state compilations, go to http://crustal.usgs.gov/namad/ Procedures and software used to compile the 1-km grid of magnetic data for Canada are detailed on the Geophysical Data Centre website http://gdcinfo.agg.nrcan.gc.ca 2002 Creation of a wavelength filtered grid. Because wavelengths greater than roughly 150 km are unreliable in the compilation, applying a high-pass wavelength filter would appear to be a viable solution to remove these unreliable wavelengths. However, removing wavelengths less than 500 km from the merged grid creates artifacts, such as spurious separation of continuous anomalies. Therefore, we removed anomalies with wavelengths greater than 500 km from the merged grid to reduce the effects caused by the erroneous long wavelengths but maintaining continuity of anomalies. The correction was accomplished by transforming the merged grid to the frequency domain, filtering the transformed data with a long-wavelength cutoff at 500 km, and subtracting the long-wavelength data grid from the merged grid. 2002 An equivalent source method based on long-wavelength characterization using satellite data was used to correct for long-wavelength shifts. We produced an aeromagnetic grid in which the wavelengths longer than 500 km have been replaced by downward-continued CHAMP satellite data. Steps 0 and 6 were performed by Bob Kucks. Steps 1-4 were performed by Tiku Ravat. Step 5 was performed by Jeff Phillips. 0. The North American 1 km merged grid was decimated to 5 km. 1. This 5 km grid was converted to a 0.05 degree grid. This grid was low-pass filtered using a Gaussian filter with a 500 km cutoff, then decimated to 1 degree. 2. A joint inversion of this 1 degree low-pass aeromagnetic grid and CHAMP satellite data, with the aeromagnetic data weighted very low, produced a stabilized downward continuation of the CHAMP data. 3. The inverted data were interpolated to 0.05 degree and again low-pass filtered using the same Gaussian 500 km filter to remove 4. The low-pass grid from step 1 was subtracted from the original 0.05 degree aeromagnetic grid to create a 500 km high-pass aeromagnetic grid. This grid was added to the low-pass inverted grid from step 3 to get a corrected 0.05 degree aeromagnetic grid. 5. The corrected 0.05 aeromagnetic degree grid was projected to the DNAG projection and regridded to 5 km. This was subtracted from the decimated 5 km aeromagnetic grid to generate a 5 km correction grid. A matched filter was used to remove short-wavelength artifacts resulting from the projection and regridding process. 6. The resulting 5 km correction grid was regridded to the original 1 km grid and subtracted from the original 1 km aeromagnetic grid to generate the final 1 km corrected aeromagnetic grid. 2002 raster grid cell 3000 5000 Albers Conical Equal Area 29.5 45.5 -96.0 37.5 0 0 row and column 1000 1000 meters North American Datum of 1927 Clarke 1866 6378206.4 1.0 USmag_origmrg.grd Grid containing the residual total intensity of the Earth's magnetic field. grid cell Residual total intensity of Earth's magnetic field. The total magnetic value minus a geomagnetic reference field (GRF), which is a long-wavelength regional magnetic field. The most commonly used reference field is determined from a model developed by the International Association of Geomagnetism and Aeronomy (IAGA). The International Geomagnetic Reference Field (IGRF), is a predictive model adopted at the beginning of a model period (e.g. in 1989 for 1990-1995). After the model period, a revised definitive model is adopted, the DGRF. This is the preferred model to use for removing regional magnetic fields Nettleton, L.L., 1971, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series No. 1, p. 83-87. A description of magnetometers and how they measure the total magnetic field can be found in: Dobrin, M.B., 1976, Introduction to Geophysical Prospecting: New York, McGraw-Hill Book Company, p. 505-517. -4937.6 13262.4 nanoteslas -1.0E+32 missing data 2 nanoteslas The Geomagnetic Reference Field that was subtracted from the total magnetic value is based on a model, which was updated to the date of the survey. New models are available at five year intervals. These models have varying degrees of accuracy in determining the local reference field. USmag_hp500.grd Grid made by removing a 500-km high-pass filtered from the total intensity data. grid cell 500-km high-pass filtered grid calculated from residual magnetic grid Because wavelengths greater than roughly 150 km are unreliable in the compilation, applying a high-pass wavelength filter would appear to be a viable solution to remove these unreliable wavelengths. However, removing wavelengths less than 500 km from the merged grid creates artifacts, such as spurious separation of continuous anomalies. Therefore, we removed anomalies with wavelengths greater than 500 km from the merged grid to reduce the effects caused by the erroneous long wavelengths but maintaining continuity of anomalies. The correction was accomplished by transforming the merged grid to the frequency domain, filtering the transformed data with a long-wavelength cutoff at 500 km, and subtracting the long-wavelength data grid from the merged grid. standard -5033.9 13218.7 nanoteslas -1.0E+32 missing data USmag_CM.grd Grid made by removing a magnetic field that was calculated from satellite magnetic data by Tiku Ravat of of Southern Illinois University. grid cell An equivalent source method based on long-wavelength characterization using satellite data was used to correct for long-wavelength shifts. We produced an aeromagnetic grid in which the wavelengths longer than 500 km have been replaced by downward-continued CHAMP satellite data. Steps 0 and 6 were performed by Bob Kucks. Steps 1-4 were performed by Tiku Ravat. Step 5 was performed by Jeff Phillips. 0. The North American 1 km merged grid was decimated to 5 km. 1. This 5 km grid was converted to a 0.05 degree grid. This grid was low-pass filtered using a Gaussian filter with a 500 km cutoff, then decimated to 1 degree. 2. A joint inversion of this 1 degree low-pass aeromagnetic grid and CHAMP satellite data, with the aeromagnetic data weighted very low, produced a stabilized downward continuation of the CHAMP data. 3. The inverted data were interpolated to 0.05 degree and again low-pass filtered using the same Gaussian 500 km filter to remove 4. The low-pass grid from step 1 was subtracted from the original 0.05 degree aeromagnetic grid to create a 500 km high-pass aeromagnetic grid. This grid was added to the low-pass inverted grid from step 3 to get a corrected 0.05 degree aeromagnetic grid. 5. The corrected 0.05 aeromagnetic degree grid was projected to the DNAG projection and regridded to 5 km. This was subtracted from the decimated 5 km aeromagnetic grid to generate a 5 km correction grid. A matched filter was used to remove short-wavelength artifacts resulting from the projection and regridding process. 6. The resulting 5 km correction grid was regridded to the original 1 km grid and subtracted from the original 1 km aeromagnetic grid to generate the final 1 km corrected aeromagnetic grid. Ravat, D., Whaler, K.A., Pilkington, M., Sabaka, T., and Purucker, M., 2002, Compatibility of high-altitude aeromagnetic and satellite-altitude magnetic anomalies over Canada: Geophysics, v. 67, p. 546-554. -5090 13227 nanoteslas -1.0E+32 missing data DVD distributor: USGS Information Services mailing address

Box 25286, Building 810 Denver Federal Center

Denver CO 80225 USA 303-202-4700 www.usgs.gov/pubprod USGS Open-File Report OFR 02-0414 Although all data published in these grids have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data or related materials. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Geosoft binary grid (GRD) 2 Magnetic anomaly data: The residual total intensity of the Earth's magnetic field 34 megabytes ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/open-file-reports/ofr-02-0414/USmag_origmrg.grd ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/open-file-reports/ofr-02-0414/USmag_origmrg.grd.gi Geosoft binary grid (GRD) 2 Magnetic anomaly data: 500-km high-pass filtered grid calculated from residual magnetic grid 34 megabytes ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/open-file-reports/ofr-02-0414/USmag_hp500.grd ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/open-file-reports/ofr-02-0414/USmag_hp500.grd.gi Geosoft binary grid (GRD) 2 Magnetic anomaly data: The result of removing a magnetic field that was calculated from satellite magnetic data by Tiku Ravat of Southern Illinois University 34 megabytes ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/open-file-reports/ofr-02-0414/USmag_CM.grd ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/open-file-reports/ofr-02-0414/USmag_CM.grd.gi none 20030303 Robert P. Kucks U.S. Geological Survey mailing address

U.S. Geological Survey Box 25046 Mail Stop 964 Denver Federal Center

Denver CO 80225 USA 303-236-1405 rkucks@usgs.gov Content Standard for Digital Geospatial Metadata FGDC-STD-001-1998